CROP RESPONSETO SOIL TEST P & K AND

STARTER FERTILIZER

Larry Bundy and Todd Andraski

Introduction• Frequent reports of K deficiency.

• More often seen in no-till.

• Do current soil test K recommend-ations need modification ?

• Does starter fertilizer containing K prevent deficiencies?

Procedure• Long-term plots with wide range of

soil test K (VL to EH, 60 to 265 ppm).

• Response to NPK starter (100/9-23-30) across range of soil test K levels.

• Corn yield responses measured over 4 yr.

Soil test K interpretation for corn(Group B soils)

Soil test K Recommend.*Category (ppm) (lb K2O/acre)

V. low (VL) < 70 100Low (L) 70-90 90

Optimum (O) 91-110 60High (H) 111-150 30

Ex. high (EH) > 150 0* 151-170 bu/acre yield goal.

Procedure• 1993 to 1996.

• P and K broadcast to some plots (1993 & 1995 – spring disked & chisel plowed) to expand the range of soil test levels.

• No-till in 1994 and 1996.

Soil test K, ppm50 100 150 200 250

Rela

tive

yiel

d, %

20

40

60

80

100

120

VL L O H EH

69% 77% 93% 98% 100%

Soil K response relationship relative to current soil test interpretation ranges at Arlington, 1993 to 1996

Growing season characteristics

Year PDRM* F.F. days GDD1993 225 154 20551994 228 189 22931995 227 145 24131996 228 170 2043

* Planting dates: Apr. 30 to May 3; RM=105

Soil test K, ppm50 100 150 200 250

Yiel

d re

spon

se, b

u/ac

re

-40

-20

0

20

40

60

80

100

VL L O H EH(47) (19) (-2) (8) (1)

QRPy=343-6.82x+0.034x2

if x<100y=0 if x>100

R2=0.52 n=54

Relationship between soil test K level and yieldresponse to starter fertilizer at Arlington, 1995

Soil test K, ppm50 100 150 200 250

Yiel

d re

spon

se, b

u/ac

re

-40

-20

0

20

40

60

80

100

VL L O H EH(34) (34) (12) (7) (7)

QRPy=145-2.04x+0.0075x2

if x<136y=6 if x>136

R2=0.32 n=111

Relationship between soil test K level and yieldresponse to starter fertilizer at Arlington, 1996

Critical soil test K level, ppm100 110 120 130

Cum

ulat

ive

GDD

2000

2100

2200

2300

2400

2500

Air t

empe

ratu

re d

epar

ture

,o F

-2

-1

0

1y = 1119 - 0.84x + 0.00017x2 R2 = 0.97

19961993

1994

1995

Relationship between temperature (GDD and departure – May to September) and maximum soil test K level where yield

response to starter fertilizer occurred

Critical soil test K level, ppm100 110 120 130

Cum

ulat

ive

GDD

2000

2100

2200

2300

2400

2500

Air t

empe

ratu

re d

epar

ture

,o F

-2

-1

0

1

1996

1993

1994

1995

Relationship between temperature (GDD and departure– May to September) and maximum soil test K level

where yield response occurred

Soil test K, ppm

50 100 150 200 250

Yiel

d, b

u/ac

re

0

10

20

30

40

50

L O H VH EH

(31 bu/a) (36) (38) (38) (38)

n = 184

Relationship between soil test K level and soybean grain yield relative to current soil test interpretation ranges at Arlington, 1992

Bray P1 soil P, mg kg-1

0 100 200 300 400

Meh

lich

III s

oil P

, mg

kg-1

0

100

200

300

400

Southern silty soils n = 350y = -7 + 1.46x r2 = 0.96Eastern clayey soils n = 56y = 8 + 1.32x r2 = 0.94

Relationship between Bray P1 and Mehlich III extractable soil P (0-2 cm) for southern silty and eastern clayey soils in Wisconsin.

Summary• Results support the soil test K & P categories

used for current fert. recommendations.

• For corn, little response to increasing soil test K above 110 ppm or above 20 ppm for P.

• Frequency and size of response to starter was influenced by GDD accumulation.

• Response to starter occurred at higher soil test K levels in cooler growing seasons

Soil test K, ppm50 100 150 200 250

Yiel

d re

spon

se, b

u/ac

re

-40

-20

0

20

40

60

80

100

VL L O H EH(38) (27) (6) (6) (3)

QRPy=153-2.35x+0.0092x2

if x<128y=0 if x>128

R2=0.34 n=309

Relationship between soil test K level and yieldresponse to starter fertilizer at Arlington, 1993 to 1996

Relationship between Bray P1 and Mehlich 3 soil tests on Eastern Red Soils in Wisconsin

y = 0.62x + 2.06R2 = 0.99

0

20

40

60

80

100

0 50 100 150

Mehlich 3 P (mg kg-1)

Bra

y P1

(mg

kg-1

)

Procedure- Corn P Response

• Results only from plots with soil test K values > 135 ppm and where no broadcast P was applied in the study year.

• Range of soil test P levels (6 – 64 ppm).

• Max. yield to calculate rel. yield was the average of plots with STP>30.

Soil test P, ppm

0 20 40 60

Rela

tive

yiel

d, %

60

80

100

120

83 92 98 100 100 Mean relative yield

VL L O H EH

Soil P response relationship relative to current soil testinterpretation ranges at Arlington, 1993 to 1996

Soil test P, ppm

0 20 40 60

Yiel

d re

spon

se, b

u/ac

re

-40

-20

0

20

40

60

QRP

R2 = 0.04 n = 52

VL L O H EH

15 9 5 4 10 Mean yield response

Relationship between soil test P level and yieldresponse to starter fertilizer at Arlington, 1996

Soil test P, ppm

0 20 40 60

Yiel

d re

spon

se, b

u/ac

re

-40

-20

0

20

40

60

QRP

R2 = 0.04 n = 147 VL L O H EH

13 8 4 0 6 Mean yield response

Relationship between soil test P level and yieldresponse to starter fertilizer at Arlington, 1993 to 1996